1. Field of the Invention
The present invention relates to an organic electroluminescent device (ELD), and more particularly, to a dual panel type organic ELD and a method of fabrication thereof.
2. Discussion of the Related Art
In general, an organic ELD emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons with the holes, generating excitons, and transitioning the excitons from an excited state to a ground state. Compared to a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD because the transition of the excitons between the two states causes light to be emitted. Accordingly, the size and weight of the organic ELD can be reduced. The organic ELD has other excellent characteristics such as low power consumption, superior brightness, and fast response time. Because of these characteristics, the organic ELD is regarded as a promising display for next-generation consumer electronic applications such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabricating the organic ELD is a simple process with a few processing steps, it is much cheaper to produce an organic ELD than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. While both the passive matrix organic ELD and the active matrix organic ELD have a simple structure and are formed by a simple fabricating process, the passive matrix organic ELD requires a relatively high amount of power to operate. In addition, the display size of a passive matrix organic ELD is limited by its structure. Furthermore, as the number of conductive lines increases, the aperture ratio of a passive matrix organic ELD decreases. In contrast, active matrix organic ELDs are highly efficient and can produce a high-quality image for a large display with a relatively low power.
In the meanwhile, organic ELDs are classified into bottom emission types and top emission types according to an emission direction of light used for displaying images via the organic ELDs.
FIG. 1 is a schematic cross-sectional view of a bottom emission type organic ELD according to a related art. Referring to FIG. 1, an array element layer 14 including a thin film transistor (TFT) “T” is formed on a first substrate 12. A first electrode 16, an organic electroluminescent (EL) layer 18, and a second electrode 20 are formed over the array element layer 14. The organic EL layer 18 may separately display red, green and blue colors for each sub-pixel region. The red, green and blue sub-pixel regions constitute one pixel region, as shown in FIG. 1. Generally, separate organic materials are used to emit light of each color for the organic EL layer in each sub-pixel region. The organic ELD is encapsulated by attaching the first substrate 12 to a second substrate 28 with a sealant 26. The organic ELD includes a moisture absorbent material 22 to eliminate moisture and oxygen that may penetrate into a capsule of the organic EL layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent 22, and the filled moisture absorbent is fixed by a holding element 25.
FIG. 2 is an equivalent circuit diagram of the organic ELD according to the related art. Referring to FIG. 2, a gate line “GL” crosses a data line “DL.” A switching element “TS” is connected to the gate line “GL” and the data line “DL,” and is located at a crossing of the gate line “GL” and the data line “DL.” A driving element “TD” is electrically connected to the switching element “TS” and an organic electroluminescent diode “DEL.” A storage capacitor “CST” is formed between a driving gate electrode “D2” and a driving drain electrode “D6” of the driving element “TD,” and the organic electroluminescent diode “DEL” is connected to a power line “PL.”
When a scan signal of the gate line “GL” is applied to a switching gate electrode “S2” of the switching element “TS,” an image signal of the data line “DL” is applied to the driving gate electrode “D2” of the driving element “TD” through the switching element “TS.” The current density of the driving element “TD” is modulated by the image signal applied to the driving gate electrode “D2.” As a result, the organic electroluminescent diode “DEL” can display images with gray scale levels. Moreover, since the image signal stored in the storage capacitor “CST” is applied to the driving gate electrode “D2,” the current density flowing into the organic electroluminescent diode “DEL” is uniformly maintained until the next image signal is applied, even when the switching element “TS” is turned off. The switching element “TS” and the driving element “TD” can be formed of a polycrystalline silicon TFT or an amorphous silicon TFT. The process of fabricating an amorphous silicon TFT is simpler than the process for a polycrystalline silicon TFT.
As mentioned above, each of the red, green and blue colors is displayed in each of the sub-pixel regions, and the brightnesses of the sub-pixel regions are controlled by the current densities supplied from the driving TFTs to the organic electroluminescent layers of the sub-pixel regions. A desired color is displayed by combining the brightnesses of each sub-pixel region, which are adjusted by independently supplying different voltages to each sub-pixel region. However, this adjusting method requires a high current density for the sub-pixel regions, leading to a fast thermalization of the organic ELD, which in turn decreases the lifetime of the organic ELD.
To overcome this disadvantage, a driving method in which the same current level is applied to all the sub-pixel regions is suggested. However, since this method requires additional layers such as a hole transporting layer, a hole injection layer, an electron transporting layer and an electron injection as part of the organic electroluminescent layer, the fabrication process becomes complicated. Accordingly, it is difficult to obtain enough brightness for each color. Another method in which a higher driving voltage is applied to the organic EL layer is suggested. However, when the driving voltage becomes higher, the lifetime of the organic ELD decreases and the emission layer of the organic ELD becomes damaged.